In recent years, as design rules have become finer, the film thickness distribution of SOI layer of an SOI wafer used for fabrication of an SOI device, in particular, fabrication of an FD-SOI (Fully Depleted SOI) device has had an influence on a device fabrication process and, consequently, the transistor characteristics. In an integrated circuit, it is important to make the characteristics of the transistors forming the circuit uniform.
Therefore, in an FD-SOI device, it is important to measure the film thickness of an SOI layer of an SOI wafer accurately. While the devices have become finer and transistors having a size of several dozen nanometers are produced, as measurement of the film thickness of an SOI layer of an SOI wafer, each point measurement of film thickness by spectroscopic ellipsometry, reflection spectroscopy, or the like is generally performed, but, due to the influence of the wavelength of light used for measurement, spatial resolution has limitations to measure up to about hundreds of nanometers.
To evaluate the influence of the film thickness of an SOI layer on the SOI device characteristics adequately, it is necessary to measure the film thickness of an SOI layer with a spatial resolution of a few nanometers which is smaller than or equal to the channel size of a transistor.
Now, as an existing method for measuring the film thickness of a thin film, in Patent Document 1, for example, measuring the film thickness by irradiating a two-layer film with X-rays and measuring the characteristic X-rays generated from each film is described.
Patent Document 2 describes that, as a method for measuring the thickness of a thin film in a structure with a substrate on which a thin film is formed, a method of measurement using fluorescent X-rays is known. Furthermore, Patent Document 2 describes that the intensity of first fluorescent X-rays generated from the thin film and the intensity of second fluorescent X-rays generated from the substrate and attenuated as a result of passing through the thin film are measured at the same time.
Patent Document 3 describes that, as an explanation of a fluorescent X-ray film thickness analyzer, a thin film on a base substrate (a standard sample substrate) is irradiated with primary X-rays and the fluorescent X-rays generated from the thin film and the base substrate are detected, whereby it is possible to find out the composition and the film thickness of the thin film.    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. H05-45147    Patent Document 2: Japanese Unexamined Patent Publication (Kokai) No. 2002-213935    Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. H10-221047